In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon and/or other mineral deposits. Generally, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
Various sources of seismic energy have been utilized in the art to impart the seismic waves into the earth. As discussed further below, such sources have included two general types: 1) impulsive energy sources, such as dynamite, and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
Since the late 1950s and early 1960s, the second type of geophysical prospecting has developed, which employs a seismic vibrator (e.g., a land or marine seismic vibrator) as the energy source, wherein the seismic vibrator is commonly used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. Thus, a seismic vibrator may be employed as the source of seismic energy which, when energized, imparts relatively low-level energy signals into the earth. The seismic process employing such use of a seismic vibrator is sometimes referred to as “VIBROSEIS” prospecting. In general, vibroseis is commonly used in the art to refer to a method used to propagate energy signals into the earth over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources. The data recorded in this way is then correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by an electric motor driving sets of counter-rotating eccentric weights, but these were quickly replaced by servo-controlled hydraulic vibrator or “shaker unit” mounted on a mobile base unit. Roughly, half of today's land seismic data surveys use P-wave hydraulic vibrators for sources. Hydraulic seismic vibrators are popular, at least in part, because of the high energy densities of such devices.
Typically, the importation of energy with vibrator devices is for a preselected energization interval, and data are recorded during the energization interval and a subsequent “listening” interval. It is desirable for the vibrator to radiate varying frequencies into the earth's crust during the energization interval. In such instances, energy at a starting frequency is first imparted into the earth, and the vibration frequency changes over the energization interval at some rate until the stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the range of the sweep, and the length of time in which the generator has to sweep through those frequencies is known as the sweep time.
Vibrators typically employ a sweep generator, and the output of the sweep generator is coupled to the input of the vibrator device. The output of the sweep generator dictates the manner in which the frequency of the energization signal, which is imparted into the earth, varies as a function of time.
Several methods for varying the rate of change of the frequency of the sweep generator during the sweep time have been proposed. For example, in the case of a linear sweep, the frequency output of the sweep generator changes linearly over the sweep time at the rate dictated by the starting and stopping frequencies and the sweep time. Further, nonlinear sweeps have been proposed in which the rate of change of the frequency of the sweep generator varies nonlinearly between the starting and stopping frequencies over the sweep time. Examples of nonlinear sweeps have been quadratic sweeps and logarithmic sweeps.
In seismic surveys conducted on dry-land, a seismic vibrator imparts a signal into the earth, where the signal generally has a much lower energy level than a signal generated by an impulsive energy source; however, the seismic vibrator can generate a signal for longer periods of time. Vibrators for use in marine seismic surveying typically comprise a bell-shaped housing having a large and heavy diaphragm in its open end. The vibrator is lowered into the water from a marine survey vessel, and the diaphragm is vibrated by a hydraulic drive system similar to that used in a land vibrator. Alternative marine vibrator designs comprise two hemispherical shells or a curved shell with flat radiating piston, where the two shells are driven by an interconnected actuator and there is also some form of sealing mechanism between the two shells, so that a volumetric source is formed when the vibrator is excited. The hydraulic actuator moves the two members relative to one another in a similar manner to the movement of the reaction mass in a land vibrator. Marine vibrators are therefore subject to operational constraints analogous to those of land vibrators. Except where expressly stated herein, “seismic vibrator” is intended to encompass any seismic vibrator implementation, including any dry land or marine implementation thereof.
The seismic signal generated by a seismic vibrator is a controlled wavetrain—a sweep signal containing different frequencies—that may be emitted into the surface of the earth, a body of water or a borehole. In a seismic vibrator for use on land, energy may be imparted into the ground in a swept frequency signal. Typically, the energy to be imparted into the ground is generated by a hydraulic drive system that vibrates a large weight, known as the reaction mass, up and down. The hydraulic pressure that accelerates the reaction mass acts also on a piston that is attached to a baseplate that is in contact with the earth and through which the vibrations are transmitted into the earth. Very often, the baseplate is coupled with a large fixed weight, known as the hold-down weight that maintains contact between the baseplate and the ground as the reaction mass moves up and down. The seismic sweep produced by the seismic vibrator is generally a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range. Seismic sweeps often have durations between 2 and 20 seconds. The instantaneous frequency of the seismic sweep may vary linearly or nonlinearly with time. The ratio of the instantaneous frequency variation over the unit time interval is defined sweep rate. Further, the frequency of the seismic sweep may start low and increase with time (i.e., “an upsweep”) or it may begin high and gradually decrease (i.e., “a downsweep”). Typically, the frequency range today is, say from about 3 Hertz (Hz) to some upper limit that is often less than 200 Hz, and most commonly the range is from about 6 Hz to about 100 Hz.
The seismic data recorded during vibroseis prospecting (hereinafter referred to as “vibrator data”) comprises composite signals, each having many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. However, when the seismic vibrator data is cross-correlated with the sweep signal (also known as the “reference signal”), the resulting correlated data approximates the data that would have been recorded if the source had been an impulsive energy source.
In many implementations, vibroseis technology uses vehicle-mounted vibrators (commonly called “vibes”) as an energy source to impart coded seismic energy into the ground. The seismic waves are recorded via geophones and subsequently subjected to processing applications. Today, various sophisticated vibrator systems are available for use, including minivibes, truck-mount vibes and buggy-mount vibes, any of which may be selected for use in a given application to provide the best possible solutions to meet a specific seismic program needs.
It is known in the seismic exploration art that the higher frequencies of energization signals are attenuated to a greater degree than lower frequency energization signals, and most authorities have concluded that the attenuation of the earth in decibels is directly proportional to the frequency of the energization signal. Further, the total attenuation of any specific signal is known to be dependent upon the velocity, layering, thickness and attenuation coefficients of each layer traversed, as well as the frequency range.
Even though the earth attenuation is known to increase with increasing frequency of the energization signals, linear sweeps have been extensively used in vibrators. Techniques for emphasizing the lower amplitude higher frequency responses are well-known and have been employed to account for the attenuation applied to these higher frequency seismic signals by the earth.
Low frequencies (e.g., below 10 Hz) are of interest today due, at least in part, to increased interest in performing acoustic impedance inversion. If seismic data can be obtained that is sufficiently quiet, then the acoustic impedance inversion process can be performed, which may result in some useful geotechnical information. An additional benefit of using low frequencies is that low frequencies penetrate farther than high frequencies, and so their use may permit evaluation of the Earth's subsurface at deeper levels. Further, by including some low frequency content in the data, it may help improve the continuity of reflectors and characteristics being imaged in the subsurface under evaluation.